A liquid crystal display (LCD) is a passive display device that utilizes the modulation of light within a liquid crystal cell to form a visible image. The LCD generally consists of a layer of liquid crystal fluid (a substance which exists at a state between liquid and crystal) between two systems of electrodes. Typically, one or both of the electrodes are transparent and each of the electrode systems is resident upon a transparent substrate. The two substrates are arranged in parallel fashion so as to form a sandwich. An integrated circuit or driver is used to selectively energize portions of the electrodes while other portions remain neutral. When the driver energizes the electrodes, the liquid crystal fluid between the energized electrodes exhibits hydrodynamic turbulence and disperses light due to a change in optical characteristics such as double refraction, optical rotation, dichroism, or optical scattering. The contrast between the dispersed transmitted and/or reflected light creates the figures or characters in the display.
Liquid crystal fluids are roughly classified into three types; smectic, cholesteric, and nematic. The nematic type of liquid crystal fluid has proven to be the most useful for LCDs, finding many uses in industrial, military and consumer products. The interior surface of each of the substrates in the LCD contains a polyimide alignment layer that has a series of minute grooves that are preferentially oriented in a given direction. The liquid crystal molecules immediately adjacent to each of the plates align themselves with the direction of orientation of the alignment layer, that is, they align themselves with the minute grooves. Those molecules that are between the plates orient themselves into a helix, the ends of which correspond with the alignment direction on each plate. When the alignment directions on each plate are at right angles to each other, the helix makes one quarter of a turn which results in linearly polarized light traversing the cell being rotated through an angle of 90.degree.. The switching in normal twisted nematic (TN) displays usually has a transition time on the order of milliseconds. The liquid crystals used in TN displays are principally cyano-biphenyl, phenylcyclohexane and cyclohexylcarboxylic ester nematic liquid crystals.
In the usual construction, the cell is sandwiched between an upper and a lower polarizing plate or polarizing filter. These polarizing plates are placed on the exterior side of each of the two substrates. Assuming the axes of the polarizing plates to be at right angles to each other, incident light will then be transmitted through the cell due to the optical activity of the liquid crystal material between the plates. However, when the electric field is applied to the selected electrodes by the driver, the liquid crystal molecules lose their optical activity because the axis of the liquid crystal molecule aligns parallel to the applied electric field. Because the polarizing filters are placed at right angles to each other, as the optical activity is lost from the selected portions of the fluid, those portions become opaque, while the remainder of the cell remains transparent. Of course, if the polarizing filters are parallel, the converse takes place.
Twisted nematic (TN) type LCD's are quite popular, and they find many uses in industrial, military and consumer products. However, one drawback is that the glass used for the substrates is fragile and susceptible to breakage. Some have sought to circumvent this problem by making the substrates out of plastic. However, since plastics are not hermetic, is necessary to coat the substrate surface on the inside of the display with a barrier layer to prevent moisture intrusion into the display. If moisture intrusion occurs, the liquid crystal fluid within the cell will degrade, resulting in black spots that are visible in the display's viewing area. Typically, this problem is circumvented by employing a barrier layer of material such as silicon oxide deposited on the plastic substrate by low temperature chemical vapor deposition. After the barrier layer is deposited, the indium-tin oxide electrodes and alignment layer are deposited. Thus, an additional step and an additional layer of material are required to use plastic substrates. It would be desirable addition to the art if plastic substrates could be used without having to resort to additional processing or complexity of the display.